-LRB- CNN -RRB- -- ISON , the most closely watched comet in recent years , may be falling apart as it nears its close encounter with the sun .

Comets are giant snowballs of frozen gases , rock and dust that can be several miles in diameter . When they get near the sun , they warm up and spew out some of the gas and dirt , creating a tail that can stretch for thousands of miles . Most comets are in the outer part of our solar system . When they get close enough for us to see them , scientists study them for clues about how our solar system formed .

When ISON was first discovered , hopes were high that it might become visible to the naked eye , meaning everyone might be able see it , not just those with good telescopes who took the trouble to find it . There was talk it might even rival some of the Great Comets like Halley 's or Hale-Bopp and spread a huge tail across the sky .

But some observers on Tuesday reported online that the comet is not nearly as bright as it has been in recent days and that it may be pouring out dust .

This could mean the comet 's core , or nucleus , has `` completely disrupted , releasing an enormous volume of dust , '' NASA 's Comet ISON Observing Campaign says in its November 25 online update .

But other observers say images taken by NASA 's STEREO spacecraft are `` encouraging evidence that the comet still exists , '' Padma Yanamandra-Fisher with the ISON campaign told reporters on the campaign 's Facebook page . She added that it 's too early to tell what kind of shape the comet is in , though .

`` I believe the next couple of days will be crucial to determine the post-perihelion appearance of the comet , '' Yanamandra-Fisher said . Perihelion is the point in an object 's path that is closest to the sun .

5 things to know about Comet ISON

Whatever its final fate , she said , ISON has `` provided a wonderful window into the world of comets . The full understanding of this comet and its place in the taxonomy of comets will only come in hindsight . ''

ISON was discovered in September of 2012 by astronomers Vitali Nevski and Artyom Novichonok using a telescope near Kislovodsk , Russia , that is part of the International Scientific Optical Network -LRB- ISON -RRB- . ISON -- officially named C/2012 S -- was 585 million miles away at the time . Its amazing journey through the solar system has been chronicled by amateur astronomers and by space telescopes . NASA has even created a toolkit for ISON fans .

Confusion about its fate is n't new for ISON watchers .

`` From the moment of discovery , ISON has been a confusing , frustrating , dynamic and unpredictable object . In other words , it has been a very typical comet ! '' said Karl Battams , an astrophysicist with the Naval Research Laboratory in Washington .

The glare of the sun has blocked most ground-based observations , but NASA has a fleet of spacecraft watching as ISON plunges toward the sun . If it has n't already broken up , it will skim about 730,000 miles above its surface on Thanksgiving Day and could put on a sky show in early December when it moves out of the glare of the sun .

The comet will make its closest approach to Earth on December 26 , and , no , it wo n't hit us . But for now , we wait to learn ISON 's fate .

`` I am excited at marking the progress of this comet that has captivated the world from its discovery and the possibility of it being a Great Comet , '' Yanamandra-Fisher told CNN.com . `` I am glad that I was able to be part of its journey . ''

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Comet ISON may or may not be breaking up

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Comet could put on spectacular show if it survives

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ISON is currently flying toward the sun